Final Report for SW00-018

Utilization of Compost Made from Agricultural and Forestry Wastes for Improving the Economic and Ecological Sustainability of Agronomic Crop Production on Low Organic Matter Soils in the San Luis Valley of Colorado

Project Information

Abstract:

This study evaluated the efficacy of utilizing compost made from cull potatoes and woodchips in potato, barley, and alfalfa production. A comparison of the agronomic benefits of applying different rates of this compost was evaluated and did not have viable economic benefits. The sawdust/cull potato compost did not have the organic content and fertility benefits of other manure based composts. No differences were observed in soil nutrient retention, crop nutrient uptake, soil microbiological health and diversity, soil water holding capacity and crop water utilization, plant growth characteristics, the suppression of soil borne pathogens, and crop yield and quality.

Project Objectives:

The main objective of this study is to improve water conservation and sustainability of crop production on the low organic matter soils of the San Luis Valley, Colorado. This will be accomplished through on-farm demonstrations that will examine the impact field incorporated compost made from agricultural and forestry wastes has on: 1) reducing the use of synthetic fertilizers and fungicides, by improving nutrient retention in the root zone and the health and diversity of the soil’s biomass, 2) improving water utilization, thereby reducing water and power use in center-pivot irrigation systems, 3) crop yields and costs of production for potatoes, barley, and alfalfa.

Proposal Objectives

1. Develop local end markets for agricultural and forestry wastes. Improve the sustainability of potato, barley, and alfalfa crop production in the San Luis Valley.

Demonstrate the impact that field incorporation of compost has on production through:

a. The change in the diversity of the soil’s microbiology and biomass.

b. Variations in disease levels in the crops.

c. The potential improvement in nutrient retention in the root zone.

d. The potential reduction in the use of synthetic fertilizers and pesticides.

e. The potential improvement in water utilization and associated reduction in water and electrical power use by center-pivot irrigation systems.

f. The net economic value of compost applications.

2. Dissemination of results to farmers in the San Luis Valley to demonstrate the economic and
ecological value of using compost to the long term sustainability of their operations.

Anticipated Schedule for Achieving Objectives

We are now at the conclusion of the study. Baseline soil samples were taken in late summer, 2000 and analyzed to establish the nutrient level and microbial levels at each test site. A water sample was taken from each of the center pivot systems and analyzed prior to compost applications. Compost was applied in the fall of each year of the project. The normal farming practice of the area is to do field preparation in the fall for planting the following spring. In one of the fields, however, it was more practical to apply the compost in the spring of 2002 due to wind erosion during the fall and winter. During the growing season, disease levels, crop health, water utilization and nutrient uptake were measured. Three cuttings of alfalfa were harvested for yield during each growing year. Crop yield data were collected at the end of each growing season for potato and barley crops. All data collected were summarized at the end of each growing season. In Year 2 and Year 3, summary bulletins were published in print and on the Colorado State University Web site:

http://www.colostate.edu/Depts/SLVRC/disease/INDEX.htm.

Field days at each site were held in Year 2 and 3 during the growing season. A summary of results was reported each year at potato and grain grower conferences in the San Luis Valley.

Introduction:

The main objective of this research was to improve water conservation and the sustainability of agronomic crop production on the low organic matter soils of the San Luis Valley (SLV) of Colorado, through on-farm demonstrations using various rates of compost made from agricultural and forestry wastes in typical cropping rotations of potatoes-barley and continuous alfalfa. This study examined the impact field incorporating compost has on:

1) The potential for reducing the use of synthetic fertilizers, by improving nutrient retention in the root zone and the health and diversity of the soil’s biomass,

2) The potential for improving water utilization and thereby reducing water and power use in center-pivot irrigation systems,

3) The potential for demonstrating disease suppression qualities which may help to improve the health of crops and reduce the need for fungicides, and

4) The potential positive impacts on crop yields and the economic returns of production for potatoes, barley, and alfalfa.

Two agricultural waste streams, sawdust and cull potatoes, being generated in the SLV have become problematic for their local industries. Logs harvested from the National Forests surrounding the valley are milled locally, generating sawdust for which there are very few feasible uses. In a 1997 Colorado State University (CSU) survey of western Colorado mill operators, the second most mentioned problem was that of mill residues (sawdust). Most of this sawdust has been just stacking up at locations near the mills. Potatoes are the SLV’s most economically important crop, and the foundation of the local economy. On average about 9.6% of each year’s potato crop is not marketable, due to size, appearance, or presence of disease. These cull potatoes may become particularly problematic, since the devastating disease late blight (Phytophthora Infestans) has been found in the valley. Late blight spores from cull piles of infected potatoes can be transported by air to infect the new growing crop, repeating the disease cycle. For this reason, local potato growers are looking at options such as composting as a method of eliminating those cull piles.

Research conducted in Maine has demonstrated that properly managed, hot aerobic composting of cull potatoes with sawdust will destroy disease pathogens and produce an excellent soil conditioner, with each ton of fresh compost containing 12 lbs of nitrogen, 4 lbs of phosphorus, 9 lbs of potash, 18 lbs of calcium, and about 400 lbs of organic matter. The sustainability of most soils used for crop production in the SLV would be improved by the addition of this compost. Sawdust and cull potatoes are produced in the SLV on a fairly consistent basis, in close proximity to one another, and in quantities complementary to what is needed for hot aerobic composting using the basic Maine recipe and methodology. While making the compost is logistically possible, there is no established local end market for the compost, and the SLV’s isolated location makes it cost prohibitive to ship to more distant markets. Local growers are reluctant to purchase and apply compost because they are trying to minimize production costs after several years of receiving low market prices, and they fear that they may be introducing disease into their fields through the compost. Local research has not yet been done to establish the benefits in long-term productivity gains that might be realized by improving soils with compost.

The objectives of this project are to develop local end markets for agricultural and forestry wastes while improving the sustainability of agronomic crop production in the SLV by demonstrating the impact compost applications have in typical cropping rotations of potatoes/barley, and continuous alfalfa. The project will examine and report on changes in the diversity of the soil’s microbiology and biomass, disease levels in the crops, use of fertilizers and pesticides, water utilization, nutrient retention in the root zone, and net economic value of compost applications.

A. Background Rationale
Late blight is now present in the San Luis Valley potato crop, requiring an increase in fungicides used as protection against this devastating disease. This is an additional economic burden to growers and adds a negative burden to the environment. Composting cull potatoes with locally generated waste sawdust will help to minimize the spread of late blight, by removing the culls as a spore source, while providing an excellent soil conditioner that can improve the sustainability of the soils used to produce agronomic crops in the valley.

Soils in the SLV used to produce agronomic crops are sandy and extremely low in organic (less than 0.5% OM). The water table in many areas of the valley is quite shallow, 5 to 30 feet below the surface. This creates the potential for nutrients applied to the crops to be leached through the soil to the water table, resulting in the loss of nutrients and contamination of the ground water.

Addition of organic matter through incorporation of compost would help to improve the soil structure and its nutrient and water holding capacity, reducing the potential for nutrient leaching and improving water conservation.

Nutrients present in the compost are released more gradually, through the decomposition performed by microorganisms that tend to be more active during the warm growing season, when plant’s root system are growing and able to take up the nutrients.

Compost also has been demonstrated to have disease suppression qualities, which may help to improve the health of crops and reduce the need for fungicides. This too could lead to a more sustainable system, with reduced input cost and reduced risk of water contamination.

B. Related Current Work in the Area
Disease Suppression: Research done by the University of Maine and Woods End Laboratories has shown that the heat generated by hot aerobic composting of cull potatoes and sawdust will destroy most potato pathogens. Disposal of infected cull potatoes by composting not only will help to minimize the spread of late blight, it is a more environmentally friendly disposal method than burial. Application of compost has been advocated by organic growers for many years as a means to eliminate pesticides and fumigants. Suppression of soil-borne plant pathogens by organics has been well documented by some researchers, through a phenomenon known as antagonism or amensalism.

Yield improvements in potatoes in response to field incorporation of compost have been documented in Maine and Idaho. Greg Porter at University of Maine saw increases in yields of US#1 potato after applying compost made with sawdust and cull potatoes. Dale Westermann, Soil Scientist, at the USDA-ARS, Research Farm in Kimberly, ID has also documented yield increases in potatoes and malting barley resulting from the application of composted dairy manure in combination with nitrogen fertilizer.

Recent work done with compost treated soils has shown an overall improvement for production agriculture purposes. Using an organic amendment led to an improved percentage of available water in all cases in research in New Mexico. The increase in available water was between 7% and 33%.

At Magic Valley Foods, in Rupert, ID, Richard Johnson was able to reduce the amount of water applied through the irrigation system in potato fields where compost had been incorporated.

Research

Materials and methods:

Compost applications were made annually to a set of agricultural fields over a three-year period. In each block there were four treatments with compost application during the fall of each of the three years. One treatment acted as a control with no compost application. The second treatment had 4 tons per acre of compost applied annually. The third treatment had 8 tons per acre of manure applied annually. The fourth treatment had 12 tons per acre of compost applied annually. Two different cooperators have fields in a potato/grain/potato rotation. On each of these circles, there were two replicated randomized blocks of treatments. Two different cooperators also had fields in a continuous alfalfa rotation. Each alfalfa field had one block of treatments.

All test plots were grown under center-pivot irrigation systems. Each treatment plot measured 100 feet wide by 1,300 feet long (approximately 3-acre strips) for the first year of the study. In subsequent study years, these treatment plots were reduced to 100 feet wide by 500 feet long (approximately 1-acre strips).

Two of the valley’s leading growers were selected as cooperators for the potato/barley/potato rotation part of the three year study. One is located in Rio Grande County in the center of the potato growing region. The second grower is located in Alamosa County in the eastern edge of the potato production area. Each grower had two replicated test blocks on their farm.

Two experienced and reliable alfalfa growers were also selected as cooperators for the three-year continuous alfalfa plots. One farmer was located in Alamosa County, on some of the SLV’s sandiest soil, with the lowest organic matter. The second farmer was located in Saguache County on fairly gravelly soil.

The purpose of the nutrient monitoring program was to examine the potential improvement in nutrient retention in the root zone and nutrient uptake by the crop as a response to compost applications. The hypothesis is that with larger applications of compost, there shall be more organic matter, better nutrient retention in the root zone, and an increased nutrient availability to the crop. In other agricultural fields in the San Luis Valley, a significant increase in available potassium has been noted following compost applications.

The producers normal fertilizer management program was applied to each test plot. Water samples were collected from each irrigation system to ascertain the amount of nitrogen being contributed to the system from the irrigation water. The soil and crop response to the compost applications made above the producers normal fertilization regime was measured.

Soil samples were collected from each of the four treatments in each of the six blocks in both the spring and fall of the year. A minimum of 20 cores was collected in each treatment to make up the composite sample. Lab results were analyzed by Servitech Labs. Analysis included: pH, salinity, excess lime, organic matter, nitrate, phosphorus, potassium, sulfur, calcium, magnesium, sodium, zinc, iron, manganese, copper, cation exchange capacity, and the percent of each cation that makes up the exchange capacity.

Petiole tissue samples were collected every other week from each of the blocks in which potatoes were grown. A total of five tissue samples in each treatment were taken across the course of the growing season from 80% cover to the start of senescence. Three of the tissue samples had a complete analysis performed which included: extractable nitrate, extractable phosphate, total phosphorus, potassium, calcium, magnesium, sulfur, copper, zinc, iron, manganese, and boron. A minimum of 20 petioles were collected to make up each tissue sample from each treatment. Petioles samples were only taken on those blocks that were in potatoes. No tissue samples were collected from the fields in small grains or alfalfa.

Soil Moisture Monitoring Methodology

The purpose of the soil moisture monitoring program was to examine the potential improvements in water utilization, increase in soil water holding capacity, and associated reduction in water and electrical power use by center-pivot irrigation systems as a response to compost applications. The hypothesis of the project was that increasing rates of compost application will increase the water holding capacity of the soil, which will decrease the rate of penetration of a single irrigation event but keep the soil from drying out as quickly enabling a longer irrigation interval. The producer’s normal irrigation water management program (both depth and timing) was not altered. Instead the soil moisture was monitored in each of the treatments to examine whether the rate of soil moisture depletion decreases with increasing compost applications. The soil moisture was monitored hourly in each plot using automated tensiometers.

To accomplish the goals of the project, three types of information were collected. First, the soil in each plot was characterized annually. This included a soil type/soil texture analysis and a determination of available water holding capacity and bulk density. The second type of information that was monitored was water applications. Wet events were measured using an automated rain collector in each field. The third type of information collected was the actual soil moisture in each treatment. Blocks 1, 3, and 5 were monitored with automated tensiometers. In each treatment, there were two automated tensiometers, one measuring the soil moisture in the shallow root zone and the other measuring the soil moisture in the medium depth of the root zone. In addition, there were two deep tensiometers per block, one in the control treatment and one in the high rate compost treatment, to measure the soil moisture at the bottom of the root zone. These automated tensiometers collected readings every hour.

The tensiometer readings at the three different depths allow the wetting front to be watched as it penetrates through the soil profile following water events to evaluate the efficacy of the irrigation amount. The tensiometer readings also allow the rate of soil drying, as a function of crop water use, to be watched to evaluate the efficacy of the irrigation interval.

Soil Microbiology Monitoring Methodology

Change in soil microbiology was measured over the course of the three-year project by taking composite soil samples of each test plot. The microbial foodweb assay was performed by Soil Foodweb, Inc., in Corvallis, OR, using methodology developed by Oregon State University. The method used allows the numbers, types, and activity of each important soil organism in the soil to be quickly assessed. Measurements were taken of the number of individuals or biomass of each group, type of organisms present and which type is dominant, how active the organisms are, and the relationship of soil organisms to plant available nitrogen. The samples were analyzed for total bacteria biomass, active bacteria biomass, total fungal biomass, active fungal biomass, total numbers of Amoebae, Flagellates, and Ciliates, and number of nematodes present in a gram of soil.

Disease Monitoring Methodology

Measurement of Potato Disease Levels: In Year 1 and Year, 3 visual inspections for disease incidence in potatoes were made in the compost treated plots and the control plot. The inspections were made by CSU, Research Associate, Andrew Houser with supervision of Extension Potato Specialists Rick Zink and Rob Davidson. Diseases to be monitored and reported on were:

Early Blight (Alternaria solani) – Visual ratings were made every other week, from mid-season on.

Rhizoctonia canker (Rhizoctonia solani) – visual inspection of stems and stolens were made at 40 days post planting. Visual inspection of the tubers were made at harvest.

Potato Early Dying (Verticillium dahilae) – visual inspection of the plants were taken at 70 days post planting.

Measurement of Barley Disease Levels: Harvested grain from the barley test plots were taken to the Coors grain elevator in Monte Vista and rated for disease and damage, as well as screening percentage and protein levels.

Yield Monitoring Methodology

Samples were taken to estimate the yield and quality of the crops grown under the different compost treatments. In the potato blocks, 15-foot sections of row were dug before harvest and graded for size and yield. The number of tubers and weight of tubers were measured to provide information about the harvestable yield, number of tubers, size profile, and quality considerations.

In the alfalfa and barley blocks, a small-scale combine was used to obtain yield samples from each treatment. Three alfalfa cuttings were taken during each season to obtain yield data at each of the alfalfa sites. Alfalfa samples were also analyzed to determine moisture content.

A potato yield monitor was installed on the harvester used to harvest block 1 and block 2 during 2002. Aerial photography was also obtained to provide a visual representation of each of the blocks.

Research results and discussion:

Compost Composition

Compost was applied to each of the treatments in the fall of each year. Lab analysis was performed on the compost each year to evaluate its nutrient content. On average, each ton of fresh compost used for this study contains 13 lbs of nitrogen, 23 lbs of phosphorus, 32 lbs of potash, 34 lbs of calcium, and about 350 lbs of organic matter. Total nitrogen varied by batch from 7 lbs to 22 lbs. Phosphorus varied from 13 lbs to 32 lbs. Potassium varied from 25 lbs to 38 lbs. Organic matter varied from 217 lbs to 458 lbs.

The potato compost is slightly basic, with a pH of 8.6. It is slightly salty (about 58 lbs per ton). It has a carbon to nitrogen ratio around 16. The compost also has trace levels of other micro-nutrients and heavy metals. The highest of which is 25 lbs of iron, 0.66 lbs of manganese, and 11 lbs of magnesium.

In comparison to other manure based composts used in the San Luis Valley, the potato based compost has less total nitrogen, less total potassium, less organic matter, and less total calcium. Phosphorus content is very comparable. In terms of micronutrients; zinc, copper and boron levels are very comparable to manure based composts. Iron levels and manganese levels are higher. Magnesium levels are lower. The pH of the potato based compost is more alkaline than most manure based composts commonly used in the San Luis Valley. However, the potato based compost tends to be much less sodic and salty than the manure based composts.

Soil Nutrient Monitoring Results

Each treatment was fertilized with the grower’s normal fertilization program. The fertilization practices for each block are contained in the appendices. The timing of fertilizer applications include dry broadcast applications in the prior fall, primarily to spread potassium and nitrogen; liquid banded fertilizer at planting time, primarily for nitrogen, phosphorus, and zinc; and in-season liquid injections through the irrigation system primarily to spoon-feed nitrogen. Common products used for dry broadcast include: Muriated Potash, Urea, Ammonium Sulfate, and Monoammonium Phosphate. Common products used for liquid banding include: 10-34, Thiosul, 32% UAN, and Ammoniated Zinc. Common products used for liquid injection include: Thiosul and 32% UAN blends.

The irrigation water from each of the fields was sampled annually during June to provide information about the background amounts of nutrients that are applied to each plot through the irrigation system. These results are also contained in the appendix. The irrigation water quality did not change dramatically from year to year. The water quality on Block 1 and 2 is very good. It has a medium level of nitrate, a medium salinity hazard, a low sodium hazard, low dissolved solids, a good calcium to sodium ratio, and boron and chloride will not adversely affect crop production. Similarly, the water quality on Block 3 and 4 is very good. It has a low level of nitrate, a low salinity hazard, a low sodium hazard, low dissolved solids, a good calcium to sodium ratio, and boron and chloride will not adversely affect crop production. The water quality on Block 5 is moderately saline. It has a high level of nitrate, a high salinity hazard, a medium sodium hazard, high dissolved solids, a good calcium to sodium ratio, boron will not adversely affect crop production but chloride will adversely effect salt intolerant crops. The water quality on Block 6 is very good. It has a low level of nitrate, a low salinity hazard, a low sodium hazard, low dissolved solids, a good calcium to sodium ratio, and boron and chloride will not adversely affect crop production.

Soil samples were collected from each of the four treatments in each of the six blocks in both the spring and fall of each year. Because the compost contains a lot of organic matter, potassium, and iron, it was anticipated that a soil response to these analytes would be seen in the higher compost treatments.

Each soil analyte was statistically analyzed to determine whether differences between the treatments could be discerned. A randomized block design was used such that the statistical difference between treatments could be discerned without the obfuscating effects of soil heterogeneity, crop type and cultivar, and location-specific growing conditions. As a parametric statistic, the analysis of variance method was used to “pool” from block to block the within-block information concerning the treatment differences while bypassing the between-block differences-that is, the heterogeneity in the experimental environment. As a result, the treatment comparisons can be made with greater precision. Using the Anova method, an alternative hypothesis that the mean analyte values are different from different treatments across blocks was tested against a null hypothesis that the mean analyte values for the different treatments are the same.

Behind this parametric hypothesis testing are several assumptions regarding the distribution of the data, such as an assumption that each set of observations is a random sample from a normal distribution. Because a full understanding of the underlying distributions are not known, and as such the assumptions cannot be completely validated, a non-parametric statistical method was also used. As a counterpart against the analysis of variance model, the Friedman Test was also used to compare differences between treatments.

In addition to hypothesis testing whether there were any differences between all of the treatments, each pair of treatments was also compared for statistical differences. Three different methods were used to hypothesis test the pairwise difference between treatment means, of which two methods are parametric and one method is nonparametric. These three tests include: the two sampled unequal variance Student’s T-test, the Tukey Interval test, and the Wilcoxon Signed Rank test.

In 2001, no differences between treatment levels were detected in any of the soil analytes. It appeared that the soil pH in the spring samples were significantly higher in the control treatment and in the high rate of compost treatment than in the low and intermediate rate of compost treatments. However, this difference cannot be explained physically and was not apparent in any of the other sampling time periods. Consequently, this difference is attributed to experimental error.

In 2002, no differences between treatment levels were detected in any of the soil analytes. It appeared that the cation exchange capacity in the spring samples increased with increasing compost treatments. However, this observation was not evident in any of the other sampling periods.

In 2003, both the spring and fall samples showed a significant increase in potassium with increasing compost treatments. The higher potassium levels were evident both in terms of extractable potassium and also potassium as a percent of the cation exchange capacity. The fall 2003 samples also tended to show that the highest compost application treatment had less calcium as a percent of the cation exchange capacity, perhaps indicating that more available potassium replaced the calcium on the exchange sites. The fall 2003 samples also tended to show that the control had less iron than the treatments. The results of this analysis support the hypothesis that potato compost applications will increase available potassium. However, these results were slower and less dramatic than has been noted in other fields with manure composts. Significant differences in the organic matter were not detected as expected.

Potato petiole tissue samples were collected every other week from each of the blocks in which potatoes were grown. A total of five tissue samples in each treatment were taken across the course of the growing season from 80% cover to the start of senescence. In a similar manner to the statistical analysis method described above, the differences in means in potato petiole tissue results were also compared. The difference in means between treatments were analyzed for each of the five weeks that samples were collected for the macro-nutrients of extractable nitrate and extractable phosphate. The remaining micro-nutrients were analyzed early season, mid-season, and late season. No difference could be detected in any of the potato petiole tissue nutrients between the different compost treatment plots at any time through the season.

There is no significant difference between the plant tissue nitrate levels in the different treatments.

Soil Moisture Monitoring Results

Soil samples were collected from Blocks 1, 3, and 5 each summer to characterize the soil. Three samples were collected from each treatment. The soil type was determined by sieve analysis. The water content was determined using a gravimetric method by weighing a moist sample of known volume, removing the water by oven drying, and weighing the dry sample. Soil types varied from Gravelly Sandy Loam to Sandy Loam to Sandy Clay Loam. Water holding capacities varied from 0.21 g/g to 0.36 g/g. The differences in water holding capacity and bulk density were compared between treatments using the statistical analysis methods described above. No statistically significant differences in soil water holding capacity between treatments with increasing rates of compost application were noted in of the three years of the study.

A record of the irrigation applications and rainfall events during the growing season was maintained. Wet events were measured using automated rain collectors in each field. The soil moisture for the control plot was modeled daily using a soil moisture model based upon the water inputs to the root zone and estimation of crop evapotranspiration.

Automated tensiometers measured the soil moisture deficit in each of the treatments of Blocks 1, 3, and 5 from 75% cover to senescence. Tensiometers were positioned in the shallow and medium depth of the root zone in each plot and were also located in the deep root zone in the control plot and the high rate of compost treatment. Irrometer tensiometers are soil moisture monitoring devices that measure soil matric potential. They have a sealed column of water and a ceramic porous tip that allows contact of the column of water with the soil moisture. As the soil dries, water is drawn out of the tensiometer creating a tension. This tension is measured with a pressure transducer and recorded to a data logger.

The tensiometer readings at the three different depths allow the wetting front to be watched as it penetrates through the soil profile following water events. If the hypothesis that applications of compost increase the water holding capacity is correct, then the soil moisture should stay higher in the shallow root zone in the higher compost treatments and the wetting front should not penetrate as deeply. The tensiometer readings also allow the rate of soil drying, as a function of crop water use, to be watched. If compost applications increase the water holding capacity, the soil moisture should not dry as rapidly between irrigations in the plots with higher compost applications.

No differences were noted from the tensiometer readings in terms of either the wetting front movement or the rate of soil drying. The difference in mean soil moisture between treatments was also evaluated using the statistical analysis methods described above. No statistically significant differences in mean soil moisture were measured between the compost treatments. The compost treatments in this study did not result in any significant differences in water holding capacity or soil moisture.

Soil Microbiology Results

In general, the soils in the San Luis Valley are bacterial dominated when the micrograms/gram of bacteria versus the micrograms/gram of fungi are compared. The three different crops included in this study all prefer soils that have nearly equal amounts of bacteria and fungi. All of the soils tested are low in fungi in comparison to available bacteria. All of these soils would benefit from inoculation with fungi and/or the addition of fungal foods for the existing fungi to grow and reproduce on. There was no difference in the bacteria or fungi in the treatments. There were some variations in the samples between years, but none of the treatments were significantly different from the others.

The protozoa in the samples were generally below desired ranges. The protozoa are a group of micro-organisms that help cycle nutrients. They tend to feed on bacteria and release nitrogen that is then available to the plants. As a result of the protozoa levels being low in all the samples, more fertilizer will be required to feed the plants and less nutrient cycling shall occur from the existing soil microbiology.

No differences between the compost treatments and the control were noted in any of the soil microbiological indicators that were measured, including active and total bacteria, active and total fungi, and protozoa.

Disease Incidence Results

Visual inspections for disease incidence in potatoes were made in the compost treated plots and the control plots. Diseases that were monitored and reported on include: Early Blight (Alternaria solani), Rhizoctonia canker (Rhizoctonia solani), Black Scurf, and Potato Early Dying (Verticillium dahilae). The plant vigor, number of stems, and number of stolons were also measured. A post harvest analysis was conducted to determine the severity of Rhizoctonia black scurf on the potato tubers. Ten tubers/treatments/replications were evaluated. No statistically significant differences in disease levels or potato plant growth characteristics were noted between the different compost treatments and the control in any of the test plots.

Harvested grain from the barley test plots were taken to the Coors grain elevator in Monte Vista and rated for disease and damage, as well as screening percentage and protein levels. No statistically significant differences in any of these disease or quality indicators was noted between the different compost treatments and the control in any of the test plots.

Yield Results

In 2002, the yield from Block 1 and Block 2 was measured with a potato yield monitor. The yield map is included in the appendices. The average yield in the different treatments ranged from 327 cwt/acre to 486 cwt/acre. While these yield differences are statistically significant, they tended to show a higher yield (in the low to mid 400 cwt/acre range) in the no compost and low (4 tons/acre) compost treatments and a lower total yield (mid 300 cwt/acre) in the higher compost treatments (8 tons/acre and 12 tons/acre treatments).

Test plots were also dug each year from the potato blocks to estimate the harvestable yield, number of tubers, size profile, or quality considerations from each of the treatments. There was no significant difference between the harvestable yields from any of the plots. While not significantly different, it did appear that the yield was slightly less in the 12 ton/acre compost treatments than in lesser compost treatments. There was no statistical difference in size profile, tuber numbers, or quality between the different treatments in any of the test blocks.

Similarly the alfalfa and barley test plots were evaluated for harvestable yield. There were no statistically significant differences in yield between treatments in any of the blocks.

Remote Sensing Results

The plot and field boundaries for each treatment and block were GPS’ed and the treatment boundaries were marked with stakes to make it easier for compost applications and data measurements to be made at the right location each year. Aerial photography was taken to ascertain visual differences between the plots. Agro Engineering uses a digital multispectral imaging system to capture aerial photographs from low altitude aircraft. The images have a 90 centimeter resolution. The images include traditional color infrared photographs as well as a crop health index. The crop health index is very sensitive to variations in vegetation biomass, vigor, and nutrient levels. No visual differences between the plots could be discerned from any of the aerial photographs. In 2003, the compost spreading rig’s tire tracks are evident in Block 1, as a result of the soil moisture at the time of spreading, and resulting compaction. No differences in biomass, vigor, or nutrient sufficiency could be noted from the aerial photography however.

Research conclusions:

The objectives of this project was to evaluate the nutrient availability and water retention effects that potato compost applications have in typical cropping rotations of potatoes/barley, and continuous alfalfa in the San Luis Valley of southern Colorado. The goal was to evaluate the efficacy of this product to assist in developing local end markets for agricultural and forestry wastes while improving the sustainability of agronomic crop production in the SLV.

The purpose for monitoring the plant nutrient status was to examine the potential improvement in nutrient retention in the root zone and nutrient uptake by the crop as a response to compost applications. The hypothesis was that increasing applications of compost would result in more soil organic matter, better nutrient retention in the root zone, and an increased nutrient availability to the crop. Significant increases in available potassium and iron were expected following compost applications. In 2001, no differences between treatment levels were detected in any of the soil analytes. In 2002, no differences between treatment levels were detected in any of the soil analytes. In 2003, both the spring and fall soil samples showed a significant increase in potassium with increasing compost treatments. The results of this analysis support the hypothesis that potato compost applications will increase available potassium. However no significant differences in soil organic matter, iron or other nutrients were observed. No difference could be detected in any of the potato petiole tissue nutrients between the different compost treatment plots at any time through the three seasons.

The purpose for monitoring the soil moisture in the different plots was to examine the potential improvements in water utilization, increase in soil water holding capacity, and associated reduction in water use by center-pivot irrigation systems as a response to compost applications. The hypothesis of the project was that increasing rates of compost application would increase the water holding capacity of the soil which would decrease the rate of penetration of a single irrigation event but keep the soil from drying out as quickly enabling a longer irrigation interval. No statistically significant differences in soil water holding capacity between treatments with increasing rates of compost application were noted in any of the three years of the study. No statistically significant differences in mean soil moisture were measured between the compost treatments. The compost treatments in this study did not result in any significant differences in water holding capacity or mean soil moisture, depth of penetration of irrigation events, or rate of soil moisture drying.

The purpose of assaying the soil microbiology was to examine the potential improvements to soil microbiological health and diversity resulting from compost applications. The hypothesis was that greater applications of compost would result in greater biodiversity and greater fungal numbers. No statistically significant differences in microbiological levels or biodiversity between treatments with increasing rates of compost applications were noted in the three years of the study.

The purpose of disease monitoring was to examine the differences in disease incidence resulting from different rates of compost applications. The hypothesis was that greater rates of compost applications would result in better plant growth characteristics and a suppression of soil borne plant pathogens. No significant differences in plant growth characteristics such as vigor, number of stems, number of stolons, and crop quality were noted. No significant differences in soil borne plant pathogens such as early blight, rhizoctonia, black scurf, or potato early dying were noted.

The purpose of yield monitoring was to examine any potential differences between harvestable yield and quality of crops grown with differing rates of compost. The hypothesis was that increasing rates of compost would improve crop yield and quality. No differences were evident between the different compost treatments.

Aerial photography was also taken to ascertain visual differences between the plots. These images were of very high resolution and were very sensitive to variations in crop health. No visual differences between the compost treatments could be discerned from any of the trials in terms of variations in vegetation biomass, vigor, or nutrient levels.

While it appears that the potato/sawdust compost product may increase soil potassium levels over time, no other differences in nutrient retention or soil moisture retention were observed with this product over the three years of this project.

The impacts of this research are that compost made from sawdust and cull potatoes has not been shown to be a viable product for production and sale in the San Luis Valley. This research did not find any agronomic benefits from using this product in commercial potato/grain and alfalfa production. The sawdust/cull potato based compost as it was produced did not have the fertility benefits, organic content, or microbiological benefits that have been noted in manure based composts. There has also been a potato industry concern about powdery scab, silver scurf, and nematode issues that a product made from cull potatoes might exacerbate. The industry has responded by stopping the production of sawdust/cull potato based composts in the San Luis Valley. The industry has shifted to the production of manure/straw based composts. The disposal issues for waste sawdust and cull potatoes has also become less of an issue over the past few years. The local sawmill has stopped producing lumber, reducing the volume of waste sawdust. Late Blight has not been a serious industry issue over the past few years and cull potatoes are being disposed of in other ways. Consequently, the disposal of waste potatoes has become less of a burden on the potato industry.

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Handouts were distributed at the San Luis Valley Research Center’s 2001 Field Day in August 2001 with an outline and background information on the composting project. A short summary was given on the project. An article, “Commercial composting digs in,” was also published in Ag Journal (Vol. 54, no. 37. Nov. 16, 2001. Arkansas Valley Journal) which discussed the progress of the project, what remained to be done, and the potential benefits of using compost in agricultural situations.

During 2002 a full issue of the CSU potato newsletter, Pomme de terre, Information for Colorado Potato Growers (Vol. 8, no. 1, January 2002) was devoted to a report on the compost project. In addition, field data from the compost project was included in our annual comprehensive research report to the Colorado potato industry in 2002, 2003, and 2004.

In Year 2 and Year 3, summary bulletins were published in print and on the Colorado State University Web site (http://www.colostate.edu/Depts/SLVRC/disease/2001PotatoDiseaseResearchReports.htm).

Project Outcomes

Project outcomes:

The compost product made from sawdust and cull potatoes was selling for between $20 and $30 per ton. In this research, potential agronomic benefits such as soil nutrient retention, nutrient uptake by the crop, soil microbiological content and diversity, soil water holding capacity and water utilization by the crop, disease suppression, plant growth characteristics, vigor and vegetation biomass, harvestable yield, and quality were examined. No agronomic benefits could be discerned from the use of this product. In a benefit/cost analysis, the potential benefits of this product do not appear to outweigh the cost. Consequently, this product is not economically viable for production agriculture in the San Luis Valley.

Farmer Adoption

While making the compost is logistically possible, there is no established open-end market for the compost, and the San Luis Valley’s isolated location makes it cost prohibitive to ship to more distant markets. Local growers are reluctant to purchase and apply compost unless there is a clear economic advantage because they are trying to minimize production costs after several years of receiving low market prices. They also fear that they may be introducing disease into their crop through the compost. The long-term productivity gains that can be realized by improving soils with a sawdust/cull potato based compost were not recognized by this research. Consequently, the industry has not adopted the use of sawdust/cull potato based composts in the San Luis Valley and has instead shifted to the production of manure based composts.

Recommendations:

Areas needing additional study

Utilization of a sawdust/cull potato based compost product does not appear to be economically and agronomically viable in the San Luis Valley. The industry has shifted away from the production and sale of this product. There is a large and growing interest in the use of manure based composts and compost teas. Additional research on the agronomic benefits and economic viability of manure based compost products would be of great value to agriculture producers in the San Luis Valley.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.

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